Microphytobenthic impact on benthic pelagic nutrient exchange in temperate shallow estuaries

Microphytobenthos (MPB) form a light-sensitive “nutrient filter” between sediment and bottom water in shallow coastal systems and the MPB activity is believed to have important implications for nutrient availability in such systems. We quantified this light-driven control on nutrient fluxes by analyzing benthic flux measurements from >3000 sediment cores sampled in five Danish estuaries. Light responses and the seasonality of benthic primary production (BPP), benthic nutrient fluxes and the MPB impact on these fluxes showed that MPB significantly intercepted fluxes of ammonium (NH4+), nitrate (NO3−) and dissolved silica (DSi) both under light and dark conditions. Phosphate (PO43−) fluxes were only significantly intercepted in light. The microphytobenthos had a large relative impact on nutrient fluxes during spring and a weaker impact in the autumn, despite significant photosynthetic activity of the MPB at this time of the year. The proportion of the annual sediment-water flux impacted by MPB reached maximum values of 65%, 73%, 89% and 96% for NH4+, NO3−, PO43− and DSi, respectively. The relative impact depended on the magnitude of the base fluxes and the relative impact increased with light availability at the stations above a threshold of 5% light at the seafloor. This documents that changing light regimes alter the MPB control on nutrient dynamics in shallow systems.